1. Field of the Invention
This invention relates to oximeters and in particular to improved oximeters which are essentially insensitive to ambient light, effectively immune from 60-cycle interference, and electronically less complicated than existing oximeters.
2. Description of the Prior Art
Oximeters are photoelectric devices which measure the oxygen saturation of blood. Historically, these devices were first used in clinical laboratories on samples of blood taken from patients. In recent years, non-invasive oximeters have been developed and are now widely used in intensive care units to monitor critically ill patients and in operating rooms to monitor patients under anesthesia. Early non-invasive devices relied on dialization of the vascular bed in, for example, the patient's ear lobe to obtain a pool of arterial blood upon which to perform the saturation measurement. More recently, non-invasive devices known as "pulse oximeters" have been developed which rely on the patient's pulse to produce a changing amount of arterial blood in, for example, the patient's finger or other selected extremity. See Yelderman et al., "Evaluation of Pulse Oximetry", Anesthesiology, 59:349-353 (1983), and Mackenzie, N., "Comparison of a Pulse Oximeter with an Ear Oximeter and an In-Vivo Oximeter", J. Clin. Monit., 1:156-160 (1985).
Pulse oximeters measure oxygen saturation by (1) passing light of two or more selected wavelengths, e.g., a "red" wavelength and an "IR" wavelength, through the patient's extremity, (2) detecting the time-varying light intensity transmitted through the extremity for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood using the Lambert-Beers transmittance law and the detected transmitted light intensities at the selected wavelengths.
Prior to the present invention, the patient's extremity has been exposed to the selected wavelengths sequentially, that is, a first light source, such as, a red-emitting LED, has been turned on for a period of time and then turned off, and then a second light source, such as, an IR-emitting LED, has been turned on and then off. See, for example, U.S. Pat. Nos. 4,167,331 and 4,407,290. Alternatively, it has been proposed to pass broadband light through the extremity and separate the transmitted light into two components using appropriate filters. See U.S. Pat. No. 3,998,550.
Each of these approaches leads to complex and/or expensive devices. For example, filters which are able to adequately separate IR from red light are generally expensive. Also, two light sensors, one for each wavelength, are required for the filter approach. Accordingly, with this approach, it is difficult to produce an inexpensive, disposable sensor module, as is required for operating room and other uses.
In the case of the sequential exposure approach, the apparatus must keep track of which light source is active. This involves deploying switches throughout the signal processing portion of the apparatus whose states are changed as the different sources become active. In addition, delay or "dead" times must be incorporated in the system to ensure that the measured transmittance relates to just the source which is currently active and not to a combination of the two sources. Moreover, the sources must be switched rapidly and the delay times must be kept short so that within each on-off/on-off cycle, the amount of blood and other characteristics of the patient's extremity remain essentially constant.
In addition to the foregoing, both approaches suffer from interference problems due to ambient light and 60-cycle power sources. In particular, changing amounts of ambient red and/or IR radiation can lead to errors in the oxygen saturation measurement. Both of these radiations are normally present in, for example, an operating room as a result of general lighting and IR heating devices. Variations in the levels of these radiations at the location of the oximetry sensor can result from such simple activities as movement of personnel or equipment within the operating room. Moreover, even constant amounts of these background radiations pose problems for existing oximeters since they can saturate the sensor and/or lead to low signal to noise ratios.
In an attempt to deal with the ambient radiation problem, existing oximeters have incorporated complicated circuitry to compensate for background radiation and have placed the sensors in hoods or other packages designed to minimize the amount of ambient light which can reach the sensor element. Notwithstanding these extensive efforts, as has been reported in the medical literature, the oximeters in use today can give false readings or can completely fail to function due to ambient radiation. See Brooks et al., "Infrared Heat Lamps Interfere with Pulse Oximeters," Anesthesiology, 61:630 (1984).
In addition to the ambient radiation problem, existing oximeters are also highly sensitive to 60-cycle interference. This high sensitivity is due to the fact that oximeters measure the changes in transmittance resulting from pulsatile blood flow in the patient's extremities, and the frequency content of such pulsatile blood flow ranges up to about 50-60 cycles per second. Accordingly, with the existing approaches, it is difficult to filter out 60-cycle interference using high pass filters since such filters would also filter out part of the signal being measured. To try to deal with this problem, oximeter manufacturers have shielded the sensors and the cables for the sensors. Such shields obviously increase the overall cost of the oximeter. Also, as is commonly known, complete removal of 60-cycle interference is extremely difficult to achieve with shielding, especially in the case of sensors which are attached to patients.